Electric field, polymer controlled release

At the simplest level, communication with amino acids, proteins, enzymes, antibodies, DNA, and whole cells is clearly important [158]. It is well known that biological systems can be influenced by the application of electric fields. The ICP provides us with the capability of charge injection or removal at biological interfaces due to its electronic conductivity. More interestingly, though is the redox capability of these polymers, whereby they can function as simple on-o f switches, facilitate controlled release of molecular species of interest, or direct mechanical interaction with the biological interface in the form of actuation. 

Cross-linked polymer gels that switch on and off the supply of drug or biomolecules in response to changes in temperature and/or pH have been developed [36-41], An electrically controlled DDS has also been developed. For example, insulin was entrapped in a poly(dimethylaminopropyl acrylamide) (PDMAPAA) gel and the delivery was controlled by electrical stimulation. As seen in Fig. 15, periodic on and off release of the protein molecule was obtained with numerous alternating applications of an electric field [42]. The response of the gel is quite sharp, and no leakage of the insulin during the off time was observed. Similarly, endrophonium chloride was released in an of-off pattern from poly(AMPS) copolymers with electric stimuli [43]. 

Liu and coworkers have been also designed and successfully realized a novel micro-electro-mechanical-systems (MEMS) based polymer drug dehvery microsystem. The device consists of an array of metallic contacts, able to create an uniform electric field. In particular, a hydrogel polymer matrix loaded with hematoxylin dye, as model of hydrophilic drug, has been smdied. The dehvery microsystem operated at normal body temperature (37 °C) under an applied voltage of 20 V. The release rate and dose were accurately controlled. The polymer responds to the electrical stimulus by shrinking and releases the hematoxylin dye into solution. The release of hematoxylin in the media was monitored using ultraviolet-visible spectrophotometry. 

Different drugs can be encapsulated within the hydrogel polymer matrix. The de-swelling of the polymer upon exposure to the applied electric field allows the encapsulated drug to be released from the matrix. The control of the applied voltage can be used to achieve pulsatile drug delivery. Alternatively, small volumes of drug may be continuously delivered to maintain the optimal therapeutic dose for the patient [84]. 

Electrically controlled drug release, including that of insulin, has also been investigated for polymer gels of DMAPAA [93]. When the electric field is applied to the PDMAPAA gel, insulin leaks from the gel at a constant rate, whereas the leakage ceases by stopping the applied field. 

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